An operational notion of classicality based on physical principles

TL;DR

This paper introduces a new notion of classicality called ontic-distinguishability within the ontological models framework, demonstrating that it can be tested with simple quantum systems and relates to existing classicality notions.

Contribution

It formulates a novel classicality criterion based on physical principles and shows its violation with minimal quantum systems, connecting it to known classicality violations.

Findings

Violation of ontic-distinguishability observed with two qubits.

Bounded success probability in a communication task for models satisfying this classicality.

Relation established between ontic-distinguishability and other classicality notions.

Abstract

One of the basic observations of the classical world is that physical entities are real and can be distinguished from each other. However, within quantum theory, the idea of physical realism is not well established. A framework to analyse how observations in experiments can be described using some physical states of reality was recently developed, known as ontological models framework. Different principles when imposed on the ontological level give rise to different theories, the validity of which can be tested based on the statistics generated by these theories. Using the ontological models framework, we formulate a novel notion of classicality termed ontic-distinguishability, which is based upon the physical principles that in classical theories extremal states are physical states of reality and every sharp measurement observes the state of the system perfectly. We construct a communication task in which the success probability is bounded from above for ontological models satisfying the notion of ontic-distinguishability. Contrary to previous notions of classicality which either required systems of dimension strictly greater than two or atleast three preparations, a violation of ontic-distinguishability can be observed using just a pair of qubits and a pair of incompatible measurements. We further show that violation of previously known notions of classicality such as preparation non-contextuality and Bell's local causality is a violation of ontic-distinguishability.